Get other's IP through a network

I am woundering if there is a way in WinXP to get someone's IP address if you are all on a network.
I am hosting a LAN soon, and there are a few people that have a habit of just grabbing tons and tons of files from my system, which lags me. I don't want to block the legit people, that are just grabbing a quick something, but these two guys I want to beable to block.

So, is there somehow that if I know thier computer name on the network I can then get thier IP, so I can block them with my firewall?

Using a firewall on a LAN is an EXTREMELY good way to make your comp lag, since all the accesses (like MAC-adress requests and so on) will make it work.

Just don't use this shitty Windows-folder access via network and use ftp with a secure password. If everyone shall access the ftp-server, just reduce the speed for all users.